Smoke detection system



Oct. 13, 1942. F.'c. EVANS'ETAL ,2 7

SMOKE DETECTION SYSTEM Filed Dec. 10, 1938 2 sheets-sheet 1 f iii LAMP Y VOLTAG E INVENTORS F. C.EVANS K.O. DONELIAN ATTORNEY BY 4.4M

Oct. 13,-1942Q Q E A ETAL 2,298,757

' SMOKE DETECTION SYSTEM Filed Dec.- 10, 1958 2 Sheets-Sheet 2 static fire detectors. lutely necessary to disable the air-conditioning Yntented' Oct. 13,, 1942 v s PATENT OFFICE 2,298,757 s roke DETECTION srs'rmu Francis 0. Evans, Dongan Hills, Staten Island,

and Khatchik 0. Donelisn, New York, N. Y., assignors to The American District Telegraph Company, New York, N. 1., a corporation or New Jersey Application iiecember 10, ms, Serial mousse Claims. (Cl. 111-311) This invention relates generally to light-sensitive smoke detecting systems adapted primarily for purposes of operating alarm devices or electrical release devices of various types. The invention is particularly adapted for use as a detection system-for closing dampers and stop-v ping fans in air-conditioning or ventilating systems wherein it is necessary to detect fire conditions when existing either in the air conditioning system or in or near the premises served by such system.

It has been suggested in the prior art that thermostatic lire detectors of various kinds are suitable for use in air-conditioning systems but these are found to be inadequate since the surrounding air is cooled to such an extent that thermostatic devices are very likely to be inoperative under most conditions. In order to detect flre promptly and reliably it is necessary to use light-sensitive smoke detecting devices which are sensitive enough to detect thin wisps of smoke circulating through air conditioning or ventilating systems, thereby obtaining positive and rapid operation of alarm circuits and damper-operating devices for disabling the air-conditioning system.

Air-conditioning systems operating to reduce the temperature of the air or to rapidly cir culate the air are almost certain to disable the usual sprinkler fire extinguishing systems as well as various other lire alarm systems since the conditioned air impelled at high velocities from duct outlets lowers the ambient temperature around automatic sprinkler heads and thermo- Consequently it is absoor ventilating system in order to allow the ordinary flre protection devices to operate in the this invention is to provide a light-sensitive smoke detecting system of high sensitivity wherein the sensitivity is maintained over a wide proper manner and to prevent extensive smoke damages from internal and external fires. This invention provides a new method and means whereby smoke detectors are adapted for detecting lire conditions and disabling the air-conditioning system both by shuttingdown tan motors and closing dampers throughout the duct system to prevent circulation of smoke and spread ilre. After all air circulation is stopped the ordinary fire detecting devices are able to operate in their intended fashion.

The smoke detecting system used to accompilsh the above mentioned results must be unusually sensitive and also. reliable to the extent that ordinary .fluctuations in the commercial power systems will not cause false alarms and false'operation oi dampers in the air-conditioning system. Accordingly the primary obiect oi to voltage fluctuations.

range of voltage fluctuations in the electrical supply .circuit and which provides stable supervision over an air-conditioning system.

"Another object of this invention is. to provide .a light-sensitive smoke detecting circuit wherein 'high sensitivity is maintained over a wide range of voltage fluctuation by means of a bridge circuit, a predetermined degree of unbalance in the bridge being necessary for causing operation of the load circuit.

A further object of this invention is to provide a light-sensitive smoke detecting system wherein high sensitivity is maintained over a wide range of voltage fluctuation by means of a bridge circuit containing a means for converting voltage fluctuations to current fluctuations whereby the bridge is maintained in a balanced condition despite the disproportionate change in the lightsensitive cell output due to the disproportionate change in the light output of the light source duringvoltage fluctuation.

Still another object 01 this invention is to provide a light-sensitive circuit of high sensitivity which is unaffected by a wide range of voltage fluctuation and which requiresonly. a single lightsensitive cell.

A further object of this invention is to pro- 7 video light-sensitive circuit which prevents false alarms or false operations of load circuits due Another object of this invention is to provide a light-sensitive circuit wherein trouble conditions may be supervised from a remote point.

Another object of this invention is to provide a light-sensitive circuit for use in smoke detecting systems wherein abridge coupling circuit prevents false alarms due to voltage fluctuation and wherein unbalance oi the bridge circuit in one direction will cause an alarm and unbalance in another direction will cause a trbubie indication.

A further object of this invention is to provide a light-sensitive circuit which maintains its sensitivity when the intensity of light falling upon the light-sensitive cell changes in spite of the characteristic oi boundary layer photovoltaic cells whereby their E. Mp1. falls oi! as light intensity decreases, and in spite of this falling oi! being other than a straight line function.

A still further object of this invention is to provide a new and improved sensitive circuit for compensating fluctuations in voltage, dis- "be obvious that the light source 2 tinguishing between alarm conditions and trouble conditions and which is much simpler in structure than similar compensating circuits shown in the prior art.

Further objects and advantages of this inven-= tion will be evident from consideration of the following description oi the embodiments shown in the drawings wherein:

Fig. 1 is a circuit diagram of the form of smoke detecting system;

Fig. 2 is a graph showing the relation between compensating current and light-sensitive cell output in the new and improved bridge circuit designed in accordance with this invention:

Fig. 3 is a circuit diagram of a modification of the circuit shown in Fla. 1;

Fig. 4 is a circuit diagr of a still further modification of the circuit shown in Fig. 1, in which all elements of the detecting system are properlysu m and Fig. 5 is a circuit of another modiflcation of this invention wherein supervision oi a plurality of smoke detection units is provided. As described above, the two principal problems in controlling air-conditioning systems by means of a sensitive smoke detector are prevention of preferred false operation due to voltage fluctuation and the operation of trouble alarms in the event of light-source failure or failure of any other part of the smoke detection system. The compen sation of voltage fluctuation in accordance with the invention will be described first.

Fig. 1 shows a light sensitive receiving device 5 and a light source 2, both of which are arranged in such 'a manner that the e of smoke be= tween the two elements will be detected by the light-sensitive cell i. Hie receiving cell i is of the type which generates an E. M. F. in response to light imp ging thereon and may be selenium or copper oxide cells of the conventional form, for example, those cells generally known as pho= iovoltaic-or boundary layer type cells. 'The compensating part of the circuit consists of a bridge connected between the light source and the lightsensitive cell. The bridge includes the light-sensitive cell I in one arm thereof, a balancing re= siltor I, an auxiliary source ofpotential 8, a potenticmcter 1 and any suitable means for introducing a compensating voltage which, as is shown in Fig. 1, may be a rectifier il in another arm Q thereof. A galvanometer relay I2 is connected across the arms of the bridge for the purposeof detecting unbalance of the bridge and also for the purpose or controlling auxiliary apparatus such as alarms or various elements of an airconditioning system such as dampers and tens.

Iithelightsourcelistobeconnectedtothe usual llO-volt A. C. supply circuit lay-means of transformer 3, potential for operating the bridge is; tapped across a small resistor so that said potential may be rectified in rectifier i8. Zit win he oper ated from a direct current supply iii-which case the compensating voltage may be ob 1- fire an iy without the need of rectifier I B. The r 1 Dis connected across the rectifier II for n. M41.

of introducing a compensa into the bridge 'which will vary in pith the voltage fluctuations from the lie-oi circuit and across the resistor I.

- acteristic is nearly linear, its current output also varies as the fourth power of the change in voltage. The potential across the resistor 5 is rectifled in rectifier ill and impressed across resistor 8 causing a change in the current on one arm of the bridge, but the output of the light-sensitive cell i is raised to approximately the fourth power of the change in potential across resistor 8 whereby. the bridge is unbalanced at the very time that it should be balanced. In order to overcome the above-mentioned unbalance, the battery 6 and potentiometer l are added to the usual bridge'circuit.

The operation of the bridge circuit shown in Fig. 1 can best be described with reference to Fig. 2 oi the drawings. The lines AA and A-B show the relation of compensating current to light-sensitive cell output in an ordinary bridge circuit which would not include battery 8 and potentiometer l. The straight line AA of Fig. 2 shows the variation in the compensating current as the lamp voltage across resistor 5 varies in accordance with fluctuation of the 1l0-volt source. Curve A-B in Fig. 2 shows the fluctuation of light-sensitive cell output as the light increases from lamp 2 in accordance with 'fluctuations in the -volt source. It is evident from Fig. 2 that the bridge is approximately balanced only for slight fluctuations of lamp voltage between Vi and V2. In fact, this range of compensation is entirely too small in very sensitive smoke detecting circuits of the kind necessary for use in fire protection systems, since, as shown by the graph, comparatively small fluctuations in lamp voltage will unbalance the bridge and cause a false alarm.

The characteristic curve Y-Y in Fig. 2 shows the effect of insertion of battery 6 and potentiometer l in one arm of the bridge. In the newv complished by connecting the battery 8 and potentiometer I so that at zero voltage across resistor 5 there is still a bias voltage from battery 6 in the bridge circuit, which changes the slope of the compensating current characteristic to such an extent that it corresponds very closely to the slope of the light-sensitive cell output. In other words, the voltage fluctuations across the 110- volt source have been converted tocurrent variations which vary almost exactly in proportion to the current variation of the light-sensitive cell output over a wide range. As a result of the change in current variation in the bridge, the

output of the light-sensitive cell flows through ,When the voltage scurce'fl'uctuatm. ht

emitted by source '2 will also fluctuate, but in a much greater proportion than the change in volt age; In fact, the light beam varies approtely asthe fourth power of the change in the enerselvanometer relay i2 in one direction to balance I the current flow from resistors l, 1 and 8 through the galvanometer relay in the opposite direction, the ultimate result being that the contactoi' the ealvanometer relay is maintained in a neutral position for voltage variations from VI to V4."

item the description of Fig. 2 it is evident that although the voltage source may fluctuate over a considerable range, the galvanometer relay will remain in a neutral position due to the very small differences in current flowing through it from each side of the bridge, whereby false alarms due to voltage fluctuation are prevented.

It is also necessary that devices used for fire detection be under constant supervision by a central ojiice attendant in order, to prevent failure of the alarm system owing to faulty operation of the various components of the system. In accordance with afurther feature cf the. invention, the galvanometerrelay i2 is arranged to-not only operate an alarm circuit'and control an air-conditioning system, but also to operate a trouble alarm in the event that either the operating voltage source or the light source 2 should fail, or, as will be shown later, if the lines leading to the receiver which houses the sensitive cell i should be accidentally broken. Relay ills provided with alarm contact I and trouble alarm contact l5, contact H closing a circuit through the alarm relay il upon an unbalance of the bridge when the cell I is obscured by smoke, and contact I! closing the circuit through the trouble relay l9. When the light source 2 either burns out or is extinguished due to power source failure, the output of the light-sensitive cell l becomes zero and the potential across resistor 8 also becomes zero, leaving the potential from the source 8 across potentiometer I as the only voltagewithin the bridge circuit. As is II to indicate a trouble condition.

the voltage source. Battery and potentiometer 1 now come into play to operate the relays Under certain conditions it may be desirable to protect a large area by means of a multipiicity of light sensitive cells with a separate light source associated with each, or a number of independent areas each protected by means of a light sensitive cell with a light source, all of which may be controlled by a single control unit, 1. e., a single sensitive galvanometer relay.

Also, in air-conditioning installations, practical considerations often necessitate the mounting of the light source, the receiver which houses the sensitive. cell, and the'control unit which houses the auxiliary equipment such as the sensitive galvanometer relay, rectifier, and various comevident from the drawing, the battery 6 and potentiometer 1 will energize the galvanometer relay in the opposite direction to that of the cell l The contact l8 of ,relay i2 will then engage contact ii to energizethe trouble relay II and open the associated trouble alarm circuit to cause a trouble indication at a central oilice. Ii, on the other hand, smoke passes between the light source 2 and the cell i, the output of cell' I will decrease a certain amount and unbalance the bridge circuit in the opposite direction as compared with a trouble, condition whereby contact ii of relay l2 will engage contact H to energize the alarm relay l8 and alarm I 2 to give an alarm indication at the central ing apparatus if a me should occur within the ducts or filters of the system.

Provision is also made for testing the smoke detectlngsystem in order tocheck its operative condition at regular time intervals.

A threepcsition switch iii is 'connected across two resistances II and 22 which are in turn connected in each loop of thebridge circuit. The switch 2| may be opened to remove the shuntfrom resistance 2i and simulate the passage of smoke between cell i and light source 2 by a corre--' sponding unbalance of the bridge circuit. The current in the cell arm of the bridge is reduced 9. given amount to simulate an alarm condition so that relays l2 and I! operate to sound an alarm or disable an air-conditioning system.

, The switch 20 may also be operated to open the shunt across resistor 22 thereby introducing a large amount of resistance in the voltage source side of the bridge tosimulate failure of ponents of the bridge circuit at points remote from each other. This introduces therefore the additional problem of providing a means whereby all lines interconnecting these units will be properly supervised so as to initiate a trouble signal should any or all of them be accidentally or maliciously disconnected. Such may be done as.

follows:

The fundamental equivalent of the circuit shown in Fig 1 may be seen as in Rig. 5. It will be seen that l ght sensitive cell In causes a current to flow through galvanometer relay "in direction 16 while the compensating voltage which, as is also shown in Fig. 1, is obtained by means of a rectifier II and biasing cell 6, causes a compensating current in the opposite direction as shown by arrow 11. When the emission from f the cell is is balanced by the compensating current, no current will flow through the sensitive gaivanometer relay.. Light sensitive cellsof the type described above have a characteristic such that when connected in multiple their emissions are additive, that is, each contributescurrent independently of the others. Thus any number of light sensitive cells as shown, as is, lb; ic,

etc..' may be connected in multiple and their total emission balanced by means of compen eating current obtained from the .voltage drops occurring across resistors (Math, 50, etc.) which will be in proportion to the number of light sensitive cells. i

Supervision of lines interconnecting receivers, light sources and sensitive relays is provided as follows: It will be noted that in the circuit arrangement as shown in Fig. 5, an accidental break in any one of the lines 80, Illa, Lila, 81, 821;, etc. will give a false alarm. If, however, lines I8 and I! are connected as at "d and Ila, and a break should occur in any of the lines 18a, 19a, 80, led, 8i, Ola, etc., current can flow through the sensitive relay i2 only ingthe direction as indicated by arrow 16 which causes the initiation of a trouble alarm. 'Note alsothat if either of lines 82 and 82a are broken, the sensitive relay i2 is totally disconnected from the system which leaves the relay armature free to seek its normally preset position. which is such that the armature makes on the trouble contact, so that a trouble alarm is initiated. It

w ll be evident'that all thefreii ements shown for the circuit as in Fig. 1 are applicable to the fundamental circuit shown in Fig. 5 without departing from the spirit of the invention.

A still further modification of the circuit is shown in Fig. 4 in which only one light sensitive cell is employed and in which the light sensitive cell 3] is supervised so as to initiate a trouble signal should either of its leads 44 or 45 as shown in Fig. 4 be broken.

Referring to Fig. 4 light sensitive cell 31 and resistor 38 which form one leg 01' a bridge may be housed in a separate and distinct receiver unit which may be situated at some point remote from and connected to the bridge I by means of lines 44 and 45. Cell 31 when illuminated by light source 46 causes a current to flow through resistor 30 in the direction indicated by arrow 31a; Likewise, the compensating current which is obtained as described for the circuit shown in Fig. 1 causes a current to flow through resistor 42 in the direction indicated by arrow 45a. A sensitive galvanometer relay 33 is connected across points 52 and 53 of the bridge I00 and a battery 43 is connected to points 54 and 55 0! the bridge I00. Battery 43 causes current to flow in the bridge as shown by arrows 42a and 42b. The bridge circuit I00 is completed through potentiometer 40 and by adjusting the variable tap 54 so as to vary the resistors in legs 40a and 40b, the bridge may be brought to a balance whereby no current flows through sensitive galvanometer 39.

Now should smoke or any other opaque object intercept the light beam, emission from cell 31 will decrease, thus decreasing the total current flowing through resistance 38 and upsetting the voltage relations existing in the bridge in such a direction that current will flow through galvanometer relay 39 in a direction shown by arrow 50 and cause the initiation 01' a smoke alarm as described for the circuit shown in Fig. 1. If, however, the A. C. supply to the light source fails or it the filament 01' light source 45 burns out, emission from cell 31 as indicated by 31a becomes zero as does the compensating current as indicated b the arrow 45a. The bridge would then remain balanced were it not for the fact that the biasing battery 48 causes a current to flow through the resistance 42 in a direction opposite to that indicated by arrow 45a and unbalances the bridge in "a manner which causes current to flow through galvanometerrelay 33 in the direction indicated by arrow 51. This direction of current initiates a trouble alarm as discussed for the circuit shown in Fig. 1.

Furthermore, should either 01' lines 50 or 51, which connect resistance 41 of the light source unit to the rectifier 43 in the control unit, and

across which the compensating voltage appears, become disconnected for any reason when the bridge is in its normal balance, the compensating voltage which causes current to flow in the direction indicated by 46a by means of rectifier 48 drops to zero, and likewise unbalances the voltage relations existing in bridge I 00 in such a manner that current flows through galvanometer relay 39 in the direction shown by arrow 51 and causes the initiation of a trouble alarm.

Likewise when the bridge I00 is normally balanced, if either line 44 or 45 to the receiver unit becomes disconnected, resistance 38 which constitutes one leg of bridge I00 is effectively disconnected from the bridge, thus causing an unbalance in the voltage relations in such manner that battery 43 causes current to flow through sensitive galvanometer relay 3!! in the direction shown by arrow 51 thus initiating a trouble alarm. It is important to note that for the circuits shown in Figs. 4 and 5, one and only one condition exists under which a smoke alarm is initiated and that is when the light beam from source 45 is intercepted by smoke or any opaque object, and that the rupture of any or all lines whichinterconnect the light source with the power lines or to the control unit and the receiver to the control unit causes the initiation of a distinctive trouble signal.

Several modifications of this inventicn are possible, among them being that shown in Fig. 3 of the drawings. This smoke detector is diflferent in that two light sensitive cells are used instead of one as shown in Fig. 1. The cell 30 receives light from the light source 3|, the combination being used to supervise the passage of smoke through an air conditioning duct or for any other suitable purpose. The other cell 32 is arranged to receive less illumination than cell 3| and serves as the compensating element which compensates for changes in the quantity of light due to voltage fluctuation. Cells 30 and 32 are arranged in balanced relation to one another in the arms 0! a bridge circuit. The same type of galvanometer relay I2 is used in this circuit as in the circuit shown in Fig.1. The cells 30 and 32 are connected directly to one end of the galvanometer winding and through the potentiometer 34 to the other end of the galvanometer relay winding. The battery 35 is connected across the potentiometer 34, the purpose of which will be explained later.

Galvanometer relay I2 is connected to alarm and trouble circuits in the same manner as shown in Fig. 1 and is intended to operate alarm and trouble relays such as l8 and I9 in Fig. 1 in the same manner as described above.

The cells 30 and 32 are connected in the bridge with the polarities indicated in the drawing and operate in such a manner that their outputs balance one another in the winding of relay :2 s0

that variations in power supply voltage have no effect on the relay I2 and thereby false alarms are prevented. The compensating cell 32 is arranged to receive less light-than cell 30, as stated previously, by-interposing the light slit 33 between the light source 3I and the cell 32. Since the output of cell 30 is greater than that of cell 32, a greater portion or potentiometer 34 will have to be incorporated in the bridge arm containing cell 30 in order to balance the bridge. When the light source 3b is extinguished due 'to power failure or filament failure, the outputs of the cells 30 and 32 will be reduced tc zero and their internal resistances will be equal.

It will now be evident that an unbalanced bridge results and current now flows from the plus terminal of battery 35 and divides into two. paths. The first path ccnsists of the cells 30 and 32 in series and the second path consists of the potentiometer 34. The. potential of battery 35 will be equally divided between cells 30 and 32 but will be unequally divided between the two legs of the potentiometer 34, marked 34a and 34b. It will now be obvious that a potential difference will exist across the terminals of the sensitive galvanometer relay I2 and a current will now therein in a direction opposite to that which H.-

sults in case of a smoke alarm and cause the' false alarms due to breaks in any outside wiring,

to power failure, and light source filament failure, and means for indicating such failure for aaoavsr dent that applicants have accomplished this result by'vmeansoi a simple light-sensitive circuit wherein an amplifier is not necessary and only arsingle light source and a single lightvsensitivecellmeetall requirements. Variousxrnodiflcations may beiymade in the embodying our invention without departing lrom l the :spiritiandi scope thereol and we desiremtherefore that only such limitations shall be placed thereon as are imposed by the prior art torassetiorth ,intheappended claims;

Weclaimi 1 t 1.111 combination, a source oiflight, a source :oi's fluctuatingvvoltage for energizing said source olilightpaphotocell for receiving light from said source ithrougha supervised area, the output of said cell varying in accordance with the fluctuations of said voltage, means responsive to said source oi, fluctuating voltage for producing an output corresponding to fluctuations in said voltage variable atsubstantially the same rate as the cell output in the normal operating range and means responsive to the resultants 0! said foutputa.

2. Incombination, a source of light energized by assource 1 fluctuating voltage, a photocell for qreceiving lighttiromsaid source through a supervised area, the output of said cell varying in proportion to a powerlgreater than one of said voltage,means responsive to current flow to said lightvsource to produce an output corresponding to fluctuations in said voltage. and variable at substantially the same rate as the cell output in the normal operating range; a source of con- :stant output in circuit with said celloutput and said current responsive outputwhich is substantiallyequai-to the dlflerencein absolute value of said=other outputs within said range and means responsive to the resultant of the outputs whereby variation of the lamp current givesone response and obstruction of the area deenergizes thecell toa'give an opposite response.

3.1n combination, a source of light, a source oivoltage subject to fluctuation ior energizing said sourceotlighha photocell forreceiving light from saldsource through a supervised area. the

output oisaid cell varying in proportion to a power greater than oneot said voltage, means responsive to current flow through said light \source'tor producing an output corresponding to fluctuations in said voltage and variable at sub- :stantially the same rate as the cell output in the lnormalwoperatingrange, whereby the resultant of said output is substantially constant in said range and meansresponsive. to the resultant of the outputs whereby variation of the lamp current gives one response and any obstruction of the area deenergizes the cell to give an opposite response.

operating the same to close the trouble circuit said iirst and second circuits.

5. In combination, a source oi light, a source not voltage ior energizing said source or light. a

photoelectric cellfor receiving light irom said sourcethrough asupervised area, a salvanometer relay having an alarm signal contact and a trouble signal contact, means for deriving a 4 voltage from said source of voltage and applying it to said relay to urge its armature to said alarm contact, biasing means for urging the armature of said galvanometer relay to its trouble contact, means for applying the celloutputto said relay in opposition to said derived voltage to normally position the relay armature in a neutral position and urge it' toward the trouble contact, wherebyany obstruction of said area reduces the cell output and the derived voltage operates the armature to the alarm contact, and a circuit fault which abnormally reduces said derived voltage allows said biasing means to operatethe relay armature to the trouble contact.

6. m combination, a source of light, a source or voltage for energizing said source 0! light..-

a photoelectric cell for receiving light from said source through a supervised area, switching .means comprising an armature, an alarm signal to normally position said armature in a neutral position and urge'it toward the trouble contact, whereby any obstruction or said area reduces the cell output and the derived voltage operates the armature to the alarm contact, and a circuit iault which abnormally reduces said derivedvoltage allows said biasing means'to operate said armature to the trouble contact.

'L'In combination, a source of light,a source oi voltage tor energizing said source at light, a

circuit connected at one of its ends in circuit 4. Incombination,a'flrstcircuit comprising a source of light and means for energizing said source, a second circuit comprising a light sensitive device :ior receiving light energy from said source through a supervised area and converting said light energy into electrical energy, an alarm eircuitand a trouble signal circuit, switching meansin thealarm and trouble circuits, voltage responsive means common to said first circuit and Tsaidcsecondcircuit for operating said switchmeansto close the alarm circuit-in response "to voltage change in saldsecond circuit result lingwlrom interception of light and biasing means connected to'the voltage responsive means for with said source of voltage !or deriving a voltage therefrom, a galvanometer relay having an alarm signal contact and a trouble signal contact and connected across the other-end oi said circuit, said derived voltage being applied to said relay to urge its armature to said alarm contact, biasing means for urging the armature oi said relay to said trouble contact and a photoelectric cell connected across said circuit intermediate 01 the ends thereof and receiving light irom said source across a supervised area, the output oi! said opposing said derived voltage to normally position said relay armature in a neutral position and urge it toward the trouble contact.

8. In combination, a source jot light, a source of voltage for energizing said source 'oi'light, a

circuit connected at one of its ends in circuit with said source of voltage, switching means having an 9. In combination, a source of light, a source of voltage ior energizing said source of light, a photoelectric cell tor receiving light from said (I source through a supervised area, a galvanometer relay having an alarm signal contact and trouble signal contact, means for deriving a voltage irom said source of voltage and applying it to said relay to urge its armature to said alarm contact, biasing means for urging the armature of said galvanometer relay to its trouble contact, means for applying the cell output to said relay in opposition tosaid derived voltage to normally position the relay armature in a neutral position and urge it toward the trouble contact, whereby any ,obstruction oi said area reduces the cell output means comprising an armature, an alarm signal contact and a trouble signal contact, means for deriving a voltage from said source of voltage and applying it to said switching means to urge its armature to said alarm contact, biasing means for urging said armature to said trouble contact, means for applying the cell output to said switching means in opposition to said derived voltage to normally position said armature in a neutral position and urge it toward the trouble contact, whereby any obstruction 01' said area reduces the cell output and the derived voltage operates the armature -tothe alarm contact, and a circuit i'ault which abnormally reduces said derived voltage allows said biasing means to operate said armature to the trouble contact and a second biasing means for operating said armature to close the trouble contact when the fourth mentioned means becomes faulted.

FRANCIS C. EVANS. KHATCHIK O. DQNELIAN. 

